FIG. 9 is a perspective view of a known resin-seal-type semiconductor laser device. In this laser device, a laser diode chip 1 is bonded to a sub-mount 2 (Si crystal) incorporating a photo diode 2p, which is in turn bonded to a main plate 3 formed integrally with a common post 3a. The laser diode chip 1 and the monitor photo diode 2p are respectively connected, through bonding wires 4, to two other posts 3b that are formed separately from the main plate 3. The main plate 3, common post 3a and posts 3b are cut and divided from a single lead frame. The laser diode chip 1 is covered with a translucent, end-face breaking preventive layer 5, and further sealed in a translucent seal resin 6. The main plate 3 and the posts 3a, 3b are fixed by the seal resin 6 at predetermined relative positions. The end-face breaking preventive layer 5 is provided for preventing the seal resin 6 located in the vicinity of a light-emitting end face of the laser diode chip 1 from being broken by a laser beam. The laser beam is emitted along a laser optical axis 1x in the form of an elliptic cone having an apex at the end face of the laser diode chip 1. The main plate 3 has fixed end portions 3s that protrude from the seal resin 6 to be fixed to jigs of an optical application apparatus in which the semiconductor laser device is mounted. The end portions of the posts 3a, 3b also protrude from the seal resin 6 to be connected by soldering to a lead that leads to a drive source of the laser diode chip 1 and a lead that leads to an optical output monitor circuit, respectively.
The end-face breaking preventive layer 5 is formed of silicon resin, for example. After liquid silicon is dropped so as to cover or coat the laser diode chip 1 and sub-mount 2, the liquid silicon is cured by heat in a furnace kept at 120.degree. C. to 160.degree. C.
Subsequently, the epoxy-based seal resin 6 is formed by injection molding conducted at 150.degree. C. or higher. When the seal resin 6 is cooled to room temperature after the injection molding, tensile stresses are applied due to an adhesive force between the surface of the laser diode chip 1 and the surface of the seal resin 6, since the end-face breaking preventive layer 5 has a larger coefficient of thermal expansion than the seal resin 6. FIG. 10 is a cross sectional view of the known resin-seal-type semiconductor laser device. The plane of this cross section and those of cross sectional views which will be referred to are perpendicular to the main plate including the laser optical axis 1x. The end-face breaking preventive layer 5 has the largest volume behind the laser chip 1 and above the sub-mount 2. Since the adhesive force between the end-face breaking preventive layer 5 and the seal resin 6 varies from portion to portion, peeling 5a may occur between the preventive layer 5 and the seal resin 6 where the adhesive force is smaller than the cutting force of the preventive layer 5. Also, where the adhesive force is larger than the material strength of the end-face breaking preventive layer 5, a crack 5b may be formed in the preventive layer 5 at its portion located above the monitor photo diode chip 2 and having the maximum amount of volumetric shrinkage.
The semiconductor laser device is incorporated for use in various kinds of optical application apparatus for optical discs, such as a compact disc, a laser beam printer and the like. In order to obtain stable digital signals in such applications, the laser device is required to emit a laser beam whose light intensity is stabilized with high accuracy, with the position and direction of its light-emitting point being highly accurately controlled.
The laser diode chip is secured to the monitor photo diode chip having an upper face serving as a light-receiving surface, and the monitor photo diode chip is adapted to monitor a light emitted backward from the laser diode chip. The light-emitting end face of the laser chip has a laser-beam-emitting portion having a size of about 5 .mu.m.times.1 .mu.m. The emitted monitor laser beam radiates or spreads out such that the full width at half maximum of the laser light intensity distribution has a lateral angle of about 10 degrees (as measured in the direction of the plane of the main plate) and a vertical angle of about 40 degrees (as measured in the direction perpendicular to the plane of the main plate). If a local portion of the end-face breaking preventive layer 5 peels off or separates from the seal resin 6 as described above, this peeling portion is given a larger light reflectance than the other portion of the layer 5, and therefore intensely reflects the laser beam emitted by the laser chip. When the laser diode is caused to generate a predetermine level of output in the form of light, therefore, the monitor photo diode which receives the light emitted backward provides an increased monitor current value, and is thus unable to control the light emitted forward with high accuracy. If cracks are formed in the end-face breaking preventive layer, the amount of light that enters the photo diode may vary due to multiple scattering of the light at the cracks, or interception of the monitor laser beam by the cracks, with a result of variations in the monitor current of the photo diode.